Difference in columns

Hi All,

I have a view that generates output similar to this.

select * from foo.view;

ts | size
-------------------+-----
2002-03-16 | 11
2002-03-17 | 16
2002-03-18 | 18
2002-03-19 | 12

I am trying to find the difference between the size column. So the desired
output would be

ts | size| Diff
-------------------+-----+------
2002-03-16 | 11 | 0
2002-03-17 | 15 | 4
2002-03-18 | 18 | 3
2002-03-19 | 12 | -6

I need the first column to be 0, since it will be 11-11. The second colum is
15-11. The third column is 18-15. The fourth column is 12-18.

Any thoughts about this?

TIA